1. Field of the Invention
The present invention broadly relates to binoculars and more particularly, to binoculars in which two objectives are separated with a fixed spacing (the length of a ground line) so as to change the eyepiece width.
2. Description of the Related Art
Hitherto, large binoculars, in particular, are known as binoculars whose two objectives are spaced apart at a fixed distance.
In this type of large binoculars, the Porro I-type prisms shown in FIG. 5 or the Porro II-type prisms shown in FIG. 6 are mostly used as erecting prisms for adjustments of the eyepiece width. These types of prisms inevitably require a larger prism chamber used for storing the prisms therein when compared to roof prisms for use in small binoculars, but the above type of binoculars are not usually designed to be portable, whereby the size of the binoculars present almost no problem. Additionally, the use of the Porro prisms makes it easier to manufacture the binoculars than the use of roof prisms.
Further, in addition to the above type of large binoculars provided with two objectives separated across a fixed spacing, binoculars equipped with a specific function are known: for example, binoculars having a built-in autofocus function, as disclosed in Japanese Patent Laid-Open No. 4-342212, and binoculars having a built-in shake correcting function, as disclosed in Japanese Patent Publication No. 57-37852. In the former type of binoculars, the spacing between the eyepieces (eyepiece width) is changed by rotating the image inverting prisms and the eyepieces around the optical axes of the objectives, respectively. On the other hand, in the latter type of binoculars, rhomboid prisms are added as means for varying the spacing between the eyepieces, because of the use of roof prisms which achieves substantial linearity between the optical axis along which the incident light travels and the optical axis along which the exit light travels. The roof prisms are coming into frequent use even for ordinary binoculars with a view to achieving smaller and lighter-weight binoculars.
However, the above types of conventional binoculars using the Porro prisms encounter the following problems. As illustrated in FIGS. 7 and 8, the spacing between eyepieces 33 are kept to match the average gap between the user""s eyes in order to maintain the distance between the optical axis along which a beam passes and impinges on the prism and the optical axis along which the beam passes and exits from each of the eyepieces 33 within certain values. With this arrangement, the objectives 31 should be located somewhere on the circumference having a radius indicated by R around the optical axis of the eyepiece 33, as shown in FIGS. 7 and 8. That is, the positional relationship between the objectives 31 and the eyepieces 33 and the arrangement of the prisms 32 cannot be totally flexible, but rather restricted in a certain manner. The same applies to the use of Porro II-type prisms.
Because of this problem, restrictions are further imposed on the arrangement of the optical system for use in the binoculars having a built-in shake correcting function, an autofocus function and so on, which hampers the downsizing of binoculars.
Additionally, when the Porro prisms, which require a larger prism chamber than the roof prisms, are used in binoculars provided with two objectives separated with a fixed spacing, the portions required for rotating the eyepieces and the prism chambers are enlarged, thereby hampering the ease of operation.
Accordingly, it is an object of the present invention to provide binoculars whose objectives are spaced apart at a fixed distance, in which the rotation of objectives is not employed for adjustments of the eyepiece width, and instead, a mere rotating operation of eyepiece portions can change the spacing between the eyepieces (eyepiece width), whereby the space of the entire main unit of the binoculars can be effectively utilized, thereby downsizing the overall binoculars containing possible added functions, for example, a built-in shake correcting function and an autofocus function.
It is another object of the present invention to provide binoculars in which adjustments of the eyepiece width can be made merely by the rotation of the Pechan roof prisms and the eyepieces, even with the use of roof prisms as erecting prisms, without requiring another type of optical member, such as rhomboid prisms.
It is still another object of the present invention to provide binoculars provided with objectives spaced apart at a fixed distance, in which a prism chamber for storing the Pechan roof prisms is made smaller, thereby enhancing the ease of operation.
It is a further object of the present invention to provide binoculars in which only one elastic member is required for performing easy and reliable locating and fixing of the respective prisms, which are comprised of the Pechan roof prisms, to a prism casing.